Edx-completion

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* Support ``get_child_blocks`` along with ``get_child_descriptors``.
* Switch from ``edx-sphinx-theme`` to ``sphinx-book-theme`` since the former is
deprecated

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* Add Django 3.0, 3.1 & 3.2 Support

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* Update ``get_key_to_last_completed_block`` to return ``full_block_key`` instead of ``block_key``

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* Future-proof usage of ``edx_toggles.toggles``

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* Replace reference to deprecated import path ``student.models``
with ``common.djangoapps.student.models``.
* Updated the build status badge in README.rst to point to travis-ci.com instead of travis-ci.org

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* Remove soon-to-be-deprecated WaffleSwitchNamespace class instances
* BACKWARD INCOMPATIBLE: Removes ``waffle()``, which returned a (now deprecated) WaffleSwitchNamespace. This should only affect tests in edx-platform.
* Requires edx-toggles>=1.2.0, which introduces a new API to waffle objects.
* Refactors ``ENABLE_COMPLETION_TRACKING_SWITCH`` from a ``LegacyWaffleSwitch`` to the updated ``WaffleSwitch``. We don't expect uses of this updated switch to require changes, unless there are surprise uses of deprecated methods from ``LegacyWaffleSwitch``.